1. Field of the Invention
The present invention relates to an oxide superconductor with a structure contrived to mitigate loss during AC (Alternating Current) applications and fabricating method thereof. In particular, it relates to an improvement of technology to mitigate the AC loss with respect to a type of the oxide superconductor, which has an oxide superconducting layer on a base material, by employing a structure that plurally divides the oxide superconducting layer.
Priority is claimed on Japanese Patent Application No. 2005-334988, filed on Nov. 18, 2005, the content of which is incorporated herein by reference.
2. Description of the Related Art
Superconducting wires are used for DC applications and AC applications. In cases where superconducting wires are used in AC applied equipments such as armature coils of motors and transformers, since superconductors called Type 2 Superconductors are used, entry of magnetic flux into the superconductor is partially allowed, accordingly AC loss inevitably occurs. As a technology for mitigating the AC loss, in the metallic superconducting wire rod or compound oxide superconducting wire, which have been practically applied, the superconducting wire is downsized and formed into a tiny superconducting filament with multifilamentary structure in an extrusion process, drawing process, or rolling process, as well as locating a high-resistance barrier layer or the like between superconducting filaments, when creating the multifilamentary structure, to make the resistance between the superconducting filaments higher. (See: J. Yoo, J. Ko, H. Kim and H. Chung, “Fabrication of Twisted Multifilamentary BSCC02223 Tapes by Using High Resistance Sheath for AC Application,” IEEE Transactions on Applied Superconductivity, 9(2): 2163-2166 (June 1999))
Instead of the metallic or compound superconductors that have been used, research and development on the oxide superconductors with a higher critical temperature has been progressed. As a part of this development, with respect to the oxide superconductor which has the oxide superconducting layer on the base material, the present inventors are pursuing with development of wire used for AC uses such as motors, transformers or the like.
Conventional metallic oxide superconductors allow metal processing and have allowed for multifilamenting by deformation processing. However, since the oxide superconductors are a type of ceramic and are extremely brittle, deformation processing is not applicable; accordingly it is necessary to downsize with a completely different technique.
Conventionally, as an example of a technique to downsize the oxide superconductor which has the oxide superconducting layer on the base material, as shown in FIG. 9, by forming an oxide superconducting layer 101 into plurally divided superconducting layers 103 by irradiating a laser beam on the oxide superconducting layer 101 formed on a tape-shaped base material 100 from its top face in the lengthwise direction, and forming a plurality of dividing grooves 102, which are made in lengthwise direction on the oxide superconducting layer 101, in the widthwise direction on the oxide superconducting layer 101.
For example, it is known that when the superconductor is divided in two pieces in the widthwise direction, the travel amount of the magnetic flux entering into the superconductor is reduced, thereby AC loss is mitigated.
For example, as an approximation formula of AC loss, the following formula (1) is known.W (AC loss)=(α/γ)×Bmγ×(w/n)  (1)
In this approximation formula, α and γ are pin parameters, Jc=αBγ−1 (Irie-Yamafuji model), α=1.644×109, γ=0.57, Bm is magnetic field amplitude, and is assumed to be in a range sufficiently larger than the equivalent center magnetic field range. Also, n shows the number of divisions of the superconductor.
That is, from the approximation formula, the value of W (AC loss) is related to the number of divisions of the superconductor. For example, it is said that when the number of divisions of the superconductor is set to 2, AC loss is ½, and when the number of divisions of the superconductor is set to 4, AC loss is ¼.
From this background, the inventors formed the oxide superconducting layer 101 as shown in FIG. 9 on the base material 100, and separated into two of divided superconducting layers 103, and then conducted tests to investigate effects of the divided structure by measuring the resistance value of the oxide superconductor which has the divided structure.
FIG. 10 shows test results for the case where the oxide superconducting layer, which is not divided, is formed on the base material and four measurement terminals (T1, T2, T3 and T4) are attached at fixed intervals on the surface in the widthwise direction, and resistance measurement is conducted by the DC four probe method. Of the four terminals, two outside terminals T1 and T4 correspond to the terminals for DC measurement, and the two inside terminals T2 and T3 correspond to the terminals for voltage measurement.
The horizontal axis of the graph of FIG. 10 shows temperature when the oxide superconducting layer of the aforementioned structure is cooled, and the vertical axis of which shows resistance values. It is clear that resistance values abruptly decline in the 80 K to 90 K temperature range, and that a transition occurs to a superconducting state. In less than 80 K temperature range, the plot of resistance values randomly fluctuates, showing a state that noise is detected after transition to a superconducting state has occurred.
FIG. 11 shows a measurement result of temperature dependency of resistance values for a sample where, with respect to the oxide superconducting layer 101, one dividing groove 102 is formed by laser in the lengthwise direction of the oxide superconducting layer 101 in the area between terminals T2 and T3, dividing the oxide superconducting layer 101 into two pieces.
In order to divide by laser, as shown in FIG. 12, a method was used where laser is irradiated from an approximately right angle (in the normal direction) from the top face of the oxide superconductor 105, and the oxide superconductor is moved in the lengthwise direction after setting a laser power so that the laser penetrates the oxide superconducting layer and reaches to the base material (see the arrow indicating the conductor movement direction).
As shown in the measurement result of FIG. 11, as the temperature of the oxide superconducting layer 101 divided into two pieces by the one dividing groove 102 is lowered, the resistance values drop by one step in the 80 K and 90 K temperature range compared to the higher temperature range, but resistance values in the same order as the resistance values prior to the drop are observed even after the resistance values have dropped. That is, approximately constant resistance values are observed in the 60 K to 80 K temperature range, which are low resistance values, but not a state that noise of resistance values is detected as shown in FIG. 10.
The inventors estimate that this is due to the following reasons.
As shown in FIG. 12, a laser beam 106 is irradiated vertically downward against the oxide superconductor 105 that is horizontally disposed, and the oxide superconducting layer of the oxide superconductor 105 is divided, and an enlarged view of the sectional observation by EPMA of the resultant oxide superconductor 105 is shown in FIG. 13.
As shown in FIG. 13, in the portion where the oxide superconducting layer is fusion cut by the laser, a molten coagulation part that projects toward the base material surface side could be confirmed. FIG. 13 shows a longitudinal cross-section which is parallel to the lengthwise direction of the base material, and the white part of FIG. 13 shows the base material. The oxide superconducting layer exists both to the left and right of the molten coagulation part which projects upward from the white part but the oxide superconducting layer is concealed in the black background part in FIG. 13, and the molten coagulation part is formed and projects along the portion which is fusion cut by the laser. It is estimated that, in the case where the oxide superconducting layer is irradiated by laser in order to fusion cut the superconducting layer as shown in FIG. 12, since the oxide superconducting layer has a thickness of 1 μm and the base material has a thickness of 100 μm, the laser reliably fusion cut a part of the base material, the portion of the base material melted by the laser becomes dross and is retained inside the dividing groove without being removed. As this molten coagulation is formed to become a projecting structure, the structure becomes such that both the left and right sides of supposedly divided oxide superconducting layer are bridged by this molten coagulation, and this part becomes an electric current pass, and thereby becomes the source of the occurrence of the low resistance values.
For example, in a case where an oxide superconductor, which has a structure that laminates a lower intermediate layer composed of Gd2Zr2O7 (abbreviation: GZO), an upper intermediate layer composed of CeO2, and the oxide superconducting layer composed of Y1Ba2Cu3Ox (abbreviation: YBCO) on a substrate of HASTELLOY (product name of Haynes International, Inc.) of Ni alloy, is used, when the projecting structure of the molten coagulation is formed as shown in FIG. 13, since the projecting structure contains a large amount of Ni, a circuit configuration like the equivalent circuit shown in FIG. 14 is made between terminals T2 and T3 such that the resistance of the molten coagulation whose main component is Ni is inserted in parallel into the resistance serial circuit loop, which is made of YBCO—CeO2—GZO-HASTELLOY substrate-GZO—CeO2—YBCO. Accordingly, it is estimated that the resistance of the molten coagulation, which has Ni as its primary component, is the cause of occurrence of the resistance values which is shown in FIG. 11.
Consequently, there has been a problem that it is not possible to fabricate the oxide superconductor which has low AC loss if the oxide superconducting layer is intended to be scribed by laser.
Instead of laser scribing, it is also conceivable that dividing groove is physically made by a cutter to divide the oxide superconducting layer to make the filament conductors. However, there is a problem in cutting accuracy by using the cutter, and there is a problem that it is not easy to cut only the oxide superconducting layer without cutting the base material, nor to form dividing grooves of uniform width in parallel to the entire length of an elongated oxide superconductor to divide into filament conductors with high transverse resistance values. Moreover, since cutting by a cutter accompanies physical load, there is a risk of providing physical damage to the base material and the oxide superconducting layer, which may deteriorate superconductivity.
The present invention was made in light of the foregoing circumstances, and its purpose is to offer a structure capable of enhancing the insulation properties of mated individual filament conductors that have been divided and of having the oxide superconductor with low AC loss, even when a structure, which divides the oxide superconducting layer into a plurality of filament conductors by irradiating laser against the oxide superconductor having a structure which disposes the oxide superconducting layer on the base material, is employed.